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We investigated whether the strength of oscillations in common synaptic input was explanatory of knee extensor (KE) torque signal complexity during fresh and fatigued submaximal isometric contractions, in adults aged from 18 to 90 years. The discharge times of motor units were derived from the vastus lateralis muscle of 60 participants using high‐density surface EMG, during 20 s isometric KE contractions at 20% of maximal voluntary contraction, performed before and after a fatiguing repeated isometric KE contraction protocol at 60% of maximal voluntary contraction. Within‐muscle coherence Z‐scores were estimated using frequency‐domain coherence analysis, and muscle torque complexity was assessed using multiscale entropy analysis and detrended fluctuation analysis. Alpha band (5–15 Hz) coherence was found to predict 23.1% and 31.4% of the variance in the complexity index under 28‐scales (CI‐28) and detrended fluctuation analysis α complexity metrics, respectively, during the fresh contractions. Delta, alpha and low beta band coherence were significantly increased due to fatigue. Fatigue‐related changes in alpha coherence were significantly predictive of the fatigue‐related changes in CI‐28 and detrended fluctuation analysis α. The fatigue‐related increase in sample entropy from scales 11 to 28 of the multiscale entropy analysis curves was significantly predicted by the increase in the alpha band coherence. Age was not a contributory factor to the fatigue‐related changes in within‐muscle coherence and torque signal complexity. These findings indicate that the strength of alpha band oscillations in common synaptic input can explain, in part, isometric KE torque signal complexity and the fatigue‐related changes in torque signal complexity. What is the central question of this study?Can the strength of oscillations in common synaptic input explain the complexity of isometric knee extensor torque signals?What is the main finding and its importance?Stronger alpha band oscillations in common synaptic input were associated with isometric knee extensor torque signals of higher complexity. Fatigue‐related changes in the torque signal complexity were associated with the fatigue‐related changes in the strength of alpha band oscillations in common synaptic input. These findings suggest that the complexity measures derived from isometric knee extensor torque signals reflect the neural mechanisms underpinning the modulation of torque control. [ABSTRACT FROM AUTHOR]

Restrictions to control the COVID-19 pandemic have caused older adults to stop their usual activities, including physical exercises. The novel approach of isometric no-load resistance training (NLRT) can be an interesting alternative to conventional training to oppose the harmful effects of detraining. We described the design and preliminary evaluation of an eight-week, twice-weekly NLRT program for older adults returning to strength training programs after COVID-19 lockdown. An older woman (66 years, 61.9kg, 158.5cm) and an older man (66 years, 84.1kg, 166.5cm) who were engaged in conventional strength training programs before the first COVID-19 lockdown participated in this case study. We collected muscle thickness measures using a B-mode ultrasound imaging and maximum isometric torque using an isokinetic dynamometer. Our results revealed that NLRT seems to be a good alternative to increase muscle thickness of knee and elbow flexors and extensors muscles in older adults. However, NLRT effects were inconsistent for maximum torque. [ABSTRACT FROM AUTHOR]

Patellofemoral pain (PFP) patients often show an altered lower limb alignment during the single-leg squat (SLS). There is evidence that proximal and distal-to-the-knee muscle alterations can modify the lower limb alignment in PFP patients. However, we observed a lack of studies investigating the possible association between the thickness and strength of proximal and distal-to-the-knee muscles and lower limb alignment during SLS in women with PFP. Therefore, this study aimed to investigate the association between the thickness and strength of lower limb muscles and dynamic knee valgus (DKV) during SLS in women with PFP. Cross-sectional study, where fifty-five women with PFP were submitted to the following evaluations: (1) muscle thickness (MT) of Gluteus Medius (GMed), Gluteus Maximus (GMax), Vastus Lateralis (VL), and Tibialis Anterior (TA); (2) isometric peak torque of hip abductors, hip external rotators, knee extensors, and foot inversors; and (3) DKV during SLS. There was a significant negative association between GMax's MT and DKV (r = −0.32; p = 0.01), and between TA's MT and DKV (r = −0.28; p = 0.03). No significant correlations were observed between isometric torques and DKV. Regression analysis found that GMax's MT explained 10% of the DKV's variance during SLS. Poor lower limb alignment during SLS is weakly associated with proximal and distal-to-the-knee muscle thicknesses, with no association with isometric torque in PFP women. Our results suggest that other factors besides strength and muscle thicknesses may explain and improve lower limb alignment in women with PFP. • Proximal and distal muscles influence dynamic tasks in patellofemoral pain syndrome. • Proximal and distal muscles' thicknesses is negatively associated with lower limb alignment. • Muscle strength is not associated with lower limb alignment in patellofemoral pain. • Gluteus maximus' thickness explained 10% of single-leg squat's dynamic knee valgus. [ABSTRACT FROM AUTHOR]

Abstract The paper deals with the torques of external muscles acting on the upper ankle joint under weight-bearing conditions and their importance in diagnosing and treating the human foot. Experimental data were collected and calculations were performed. Based on the experiments with the biomechanical model of the foot and upper ankle joint, it was shown how the changes in the force arms of the external muscles of the foot under weight-bearing conditions, change the torque. The real values of muscle forces and torques of the external muscles of the foot were calculated. Taking into account the distance of the lines of muscle action from the axis of rotation of the upper ankle joint the rotational force of the muscles was calculated. The influence of changing the force arm on the rotational efficiency of the muscle balancing the moment of gravity was shown. Knowledge of muscle torque under weight-bearing conditions is crucial for correctly assessing foot biomechanics. It has been shown that torque (gravitational and muscular), not pure force, is crucial when assessing the rotational capacity of the analyzed joint. A change in the approach to diagnostics and treating paresis or weakness of extrinsic foot muscles was proposed through the manipulation of the distance of their action line from the axis of joint rotation.

Purpose: This study aimed to determine the contribution of metabolic, cardiopulmonary, neuromuscular, and biomechanical factors to the energy cost (ECR) of graded running in well-trained runners. Methods: Eight men who were well-trained trail runners (age: 29 [10] y, mean [SD]; maximum oxygen consumption: 68.0 [6.4] mL·min−1·kg−1) completed maximal isometric evaluations of lower limb extensor muscles and 3 randomized trials on a treadmill to determine their metabolic and cardiovascular responses and running gait kinematics during downhill (DR: −15% slope), level (0%), and uphill running (UR: 15%) performed at similar O2 uptake (approximately 60% maximum oxygen consumption). Results: Despite similar O2 demand, ECR was lower in DR versus level running versus UR (2.5 [0.2] vs 3.6 [0.2] vs 7.9 [0.5] J·kg−1·m−1, respectively; all P <.001). Energy cost of running was correlated between DR and level running conditions only (r2 =.63; P =.018). Importantly, while ECR was correlated with heart rate, cardiac output, and arteriovenous O2 difference in UR (all r2 >.50; P <.05), ECR was correlated with lower limb vertical stiffness, ground contact time, stride length, and step frequency in DR (all r2 >.58; P <.05). Lower limb isometric extension torques were not related to ECR whatever the slope. Conclusion: The determining physiological factors of ECR might be slope specific, mainly metabolic and cardiovascular in UR versus mainly neuromuscular and mechanical in DR. This possible slope specificity of ECR during incline running opens the way for the implementation of differentiated physiological evaluations and training strategies to optimize performance in well-trained trail runners. [ABSTRACT FROM AUTHOR]

A musculoskeletal (MSK) model is a valuable tool for assessing complex biomechanical problems, estimating joint torques during motion, optimizing motion in sports, and designing exoskeletons and prostheses. This study proposes an open-source upper body MSK model that supports biomechanical analysis of human motion. The MSK model of the upper body consists of 8 body segments (torso, head, left/right upper arm, left/right forearm, and left/right hand). The model has 20 degrees of freedom (DoFs) and 40 muscle torque generators (MTGs), which are constructed using experimental data. The model is adjustable for different anthropometric measurements and subject body characteristics: sex, age, body mass, height, dominant side, and physical activity. Joint limits are modeled using experimental dynamometer data within the proposed multi-DoF MTG model. The model equations are verified by simulating the joint range of motion (ROM) and torque; all simulation results have a good agreement with previously published research. [ABSTRACT FROM AUTHOR]

AbstractA musculoskeletal (MSK) model is an important tool for analysing human motions, calculating joint torques during movement, enhancing sports activity, and developing exoskeletons and prostheses. To enable biomechanical investigation of human motion, this work presents an open-source lower body MSK model. The MSK model of the lower body consists of 7 body segments (pelvis, left/right thigh, left/right leg, and left/right foot). The model has 20 degrees of freedom (DoFs) and 28 muscle torque generators (MTGs), which are developed from experimental data. The model can be modified for different anthropometric measurements and subject body characteristics, including sex, age, body mass, height, physical activity, and skin temperature. The model is validated by simulating the torque within the range of motion (ROM) of isolated movements; all simulation findings exhibit a good level of agreement with the literature. [ABSTRACT FROM AUTHOR]

Introduction: Continuous evaluations of athletes, including strength testing, can help control performance improvement or facilitate the restoration of normality after an injury. The aim of the present study was to prospectively determine the peak torque (PT), angle at which PT is achieved, and functional ratios of flexors and extensors thigh muscles during one season. Material and methods: Thirty semi-professional male athletes competing in long jumping (n = 10), javelin throwing (n = 10), and sprinting (n = 10) participated. PT was evaluated in relation to limb length; the angle at which PT was achieved was obtained from the force-curve displayed in the isokinetic dynamometer; functional ratios were calculated by dividing concentric hamstring strength by eccentric quadriceps strength (flexor ratio) or vice-versa for the extensor ratio. Assessment was performed at 60º/s and 300º/s. Results: Significant variations were seen for both extensor and flexor PTs at different stages of the season, with moderate to large effect sizes observed (effect size (d) = 0.49–0.93). Functional ratios and the angle at which peak torque was achieved remained stable throughout the season. Conclusions: Thigh muscle strength is unstable throughout a track and field season, coaches or medical staff should consider these findings when programming training sessions or rehabilitating an athlete.

There are no studies that have simultaneously examined muscle strength of high- and low-level athletes in each of the two competitive styles in wrestling. The purpose of this study was to clarify the characteristics of muscle strength by style and level of competition in lightweight athletes specializing in freestyle (FS) and Greco-Roman style (GS) in wrestling. Thirty-seven male collegiate lightweight wrestlers (age, 20.3±1.2 years; height, 165.3±3.7 cm; weight, 63.6±2.9 kg; competition career, 12.7±3.7 years) participated in this study. Participants were divided into 4 groups as follows: high-level GS (HGS, n＝9), low-level GS (LGS, n＝8), high-level FS (HFS, n＝8), and low-level FS (LFS, n＝12) . Isometric and isokinetic muscle extension and flexion torque of elbow, knee, and trunk as muscle strength were measured by biodex system 4. The main results of this study showed that isometric and isokinetic (60, 180, and 300 deg/sec) elbow extension strength were higher in high-level athletes than in low-level athletes, regardless of competitive style (all P<0.05), and that HGS was higher than HFS and LGS in isometric knee flexion muscle strength (P＝0.001, P＝0.023, respectively), and that GS was higher than FS in isometric knee and trunk extension muscle strength (P＝0.029, P＝0.014, respectively) . The results of this study suggest the importance of upper extremity extensor strength in wrestling competitions and the need for strength improvement programs based on competition style. [ABSTRACT FROM AUTHOR]

Older females experience higher rates of disability than males, potentially due to sex-specific differences in gait and muscle function. The authors evaluated the effects of age and physical activity (PA) on gait mechanics and knee extensor muscle function in males and females. Three groups of 20 individuals (each 10 females) participated: young (21-35 y) and highly and less active older (55-70 y) adults. Knee extensor strength and joint mechanics during preferred speed gait were collected before and after 30 minutes of walking. Age by sex and PA by sex interactions indicated older and less active older females had lower concentric knee extensor muscle power and larger hip extension moments than males. After 30 minutes of walking, older less active adults had larger decreases in knee extensor power than their highly active older counterparts, and older adults of both sexes had decreases in ankle dorsiflexion moments while young adults did not. These results suggest that older, particularly less active, adults are susceptible to knee extensor muscle fatigue from moderate activity. For older adults, high levels of PA may be necessary to preserve gait mechanics in response to a bout of exercise. This new information may be important for targeting interventions in at-risk older adults [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study was to compare muscle strength at 90° hip and knee flexion as measured in three different positions and to investigate whether an internal or external deficit in the range of rotation in the hip joint affects flexor muscle strength. Methods: We measured the peak muscle torque of rotation in the hip joint, using isometric torquemeter, and hip ROM in healthy participants N = 40, aged 21.6 ± 1.9, in three different measurement positions. We tested for differences between the positions, and for the potential influence of participant’s sex and ROM asymmetry. Results: The measured peak muscle torque was affected not only by sex and the value of hip flexion affect, but also by the position in which it is measured. Subjects with restricted external rotation of the hip joint (CERD) had significantly higher flexor peak muscle torque compared to subjects with restricted internal rotation (CIRD), in all but the supine position. For CERD, the results were: Supine (SuP) 1.02 ± 0.26; Sitting (StP) 1.32 ± 0.58; Standing (SP) 1.53 ± 0.47; and for CIRD, the results were: Supine (SuP) 1.05 ± 0.17; Sitting (StP) 1.05 ± 0.40; Standing (SP) 1.47 ± 0.53. Conclusions: Overall, measurement position and passive ROM significantly influence the peak muscle torque in isometric conditions. Moreover, an imbalance in thigh rotation movement significantly determines the magnitude of muscle torque of the hip flexion movement. Individuals with increased internal-toexternal rotation achieved significantly higher values for flexor muscle torque force moments. Overall, these findings are of importance for interpreting or comparing any reported values for muscle torque force moments. [ABSTRACT FROM AUTHOR]

Abstract Background Therapeutic management of the upper extremity (UE) function of people with spinal muscular atrophy (SMA) requires sensitive and objective assessment. Therefore, we aimed to measure physiologic UE function of SMA patients with different functional abilities and evaluate the relation between these physiologic measures and functional UE scales. Methods 12 male and 5 female SMA patients (mean age 42 years; range 6–62 years) participated in this explorative study. Concerning the physiologic level, the maximal muscle torque, the maximal and normalized surface electromyography (sEMG) amplitudes, and the maximal passive and active joint angles were measured. Concerning the activity level, the Performance of the Upper Limb (PUL) scale was used, and hand function was examined using the Nine-Hole Peg Test and the Timed Test of In-Hand Manipulation (TIHM). Results Outcome measures that significantly related to the functional ability were: the PUL score (all dimensions); the finger to palm task of the Timed TIHM; biceps, triceps, and forearm extensor strength; and the active range of motion of shoulder abduction, shoulder flexion, and wrist extension. In addition, the following physiologic variables were related to the activity level (PUL score): hand function (the Nine-Hole Peg Test; Rs = − 0.61), the Timed TIHM (Rs = − 0.53), the maximal muscle torque (Rs = 0.74), the maximal sEMG amplitude (Rs = 0.79), and the maximal active joint angle (Rs = 0.88). Conclusions Muscle functions in SMA patients are already affected before activity limitations are noticeable. Consequently, monitoring the maximal muscle strength and the normalized muscle activity during task performance could play a role in the early detection of UE limitations. The mechanism behind the loss of arm activities due to SMA is primarily caused by decreasing muscle capacity, which influences the ability to move an arm actively. In clinical practices, these dimensions should be considered separately when monitoring disease progression in order to better evaluate the need for interventions.

Introduction: Applying rehabilitative exercise instructions before surgery is fundamental to minimize surgery complications and rehabilitation period as well. The aim of this research was to study the effects of 8 weeks of selected exercises on knee muscle torque and proprioception among the athletes with injured meniscus.Materials and Methods: This was an applied research with quasi-experimental method. Subjects were 24 athletes with injured meniscus who were selected through convenience judgmental sampling. Based on the orthopedic surgeon’s decision and their availability, subjects were divided equally into two experimental and control groups. Subject’s muscle torque and proprioception of the injured knee was recorded at the beginning and after eight weeks of selective exercises. The intervention was performed one hour a day on daily basis. Repeated measures variance ANOVA, within the 0.05 level of significance, was used for inferential data analysis and the probable differences between group means.Results: In both groups, maximum knee flexion and extension muscle torque and proprioception changed significantly (P < 0.001 for all three variables). Moreover, between groups differences were significant (P < 0.001).Conclusion: In this study, positive effects of exercise on torque and proprioception of knee in athletes with injured meniscus were reported. It seems that applying proper exercises challenging functional system can be an effective means of progressing knee motor function preoperatively.

The study aimed to analyse the effectiveness of two variants of 8-week strength training (hypertrophy, strength) with different modes of resistance. Healthy male subjects (n=75) were allocated to five groups of equal size: hypertrophy training with a variable cam (Hyp-Cam), hypertrophy training with disc plates (Hyp-Disc), maximal strength training with a variable cam (Str-Cam), maximal strength training with disc plates (Str-Disc), and a control group (CG). The Hyp-Cam and Str-Cam groups trained with a machine where the load was adjusted to the strength capabilities of the elbow flexors. The Hyp-Disc and Str-Disc groups trained on a separate machine in which a load was applied with disc plates. The CG did not train. All groups were assessed for changes and differences in one-repetition (1RM) lifts, isokinetic muscle torque, arm circumference and arm skinfold thickness, and plasma creatine kinase (CK) activity. Within the 8-week training period the 1RM increased (p.05) group. Skinfold thickness of the biceps and triceps decreased more within the 8-week period in Str-Cam (by 1.1 and 2.1 cm; p

Purpose: The purpose of the present study was to determine whether there are sex differences in fatigue-induced changes in quiet standing and dynamic balance and establish whether changes in muscle torque and resting stiffness may explain the potential sex differences in balance responses. Methods: Sixteen recreationally active men (age; 24.8 ± 5.0 years, height; 178.2 ± 5.6 cm, mass; 77.8 ± 13.2 kg) and 10 women (age; 21.0 ± 1.6 years, height; 167 ± 5.3 cm, mass; 61.3 ± 8.9 kg) were assessed for postural sway, Y balance test performance, isokinetic and isometric knee extensor torque and resting stiffness of the vastus lateralis (VL), gastrocnemius lateralis (GL) and Achilles tendon (AT) before and immediately after fatiguing exercise. The fatigue protocol consisted of five sets of 20-drop jumps. Results: The fatiguing exercise elicited similar magnitude (effects size; ES) reductions in muscle torque (men; ES = 0.45–0.80, women; ES = 0.46–0.52), dynamic balance (men; ES = 0.45–0.74, women; ES = 0.47–0.79) and resting VL stiffness (men; ES = 0.46, women; ES = 0.36) in men and women (all p < 0.05). For quiet standing balance, fatigue induced an increase in postural sway metrics (ES = 0.64–1.28) and reduction in resting GL stiffness (ES = 0.40) in men (both p < 0.001) but not women (p > 0.05). Conclusion: Fatiguing exercise, when producing a similar level of force reduction, induces similar magnitude reductions in dynamic postural control and resting VL stiffness in men and women. Distinct deteriorations in quiet standing balance in men but not women were accompanied by modifications in calf muscle stiffness following exercise-induced muscle fatigue. [ABSTRACT FROM AUTHOR]

Objectives: Determine the effects of varying frequency, phase duration, and interphase interval (IPI) of symmetrical, biphasic pulsed current (SBPC) on quadriceps femoris muscle torque production, perceived discomfort, and muscle fatigue when using neuromuscular electrical stimulation (NMES). Methods: Ten recreationally active able-bodied participants completed this study. Muscle torque was measured during a series of laboratory based, NMES-induced muscle contractions with varying combinations of phase durations (50, 100, 150, or 200 µs) and IPIs (50, 100, 150, or 200 µs) at 25 and 50 Hz. A three-factorial repeated measures experimental design was used. After each contraction, participants rated perceived discomfort on a 10-cm visual analog scale. Subsequently, fatigue tests were conducted using different frequency-IPI combinations. Results: Results of within-subjects ANOVAs revealed no interaction among the three factors: frequency, phase duration and IPI. Significant main effects of phase duration and IPI, but not frequency, on torque production and perceived discomfort were observed. Specifically, significant linear and quadratic trends were observed for the effect of phase duration and IPI on torque production, and the effect of IPI on discomfort, and only a significant linear trend for the effect of phase duration on discomfort. The shorter IPI combined with the longer phase duration produced greater torque with tolerable discomfort. There was no significant effect of IPI on muscle fatigue. Conclusions: Study findings indicated that when administering SBPC on the quadriceps femoris muscle, clinicians should select an IPI of 50 µs and a longer phase duration (e.g. 200 µs) to maximize torque without imposing intolerable discomfort. [ABSTRACT FROM AUTHOR]

This study explores the cardiorespiratory and muscular fatigue responses to downhill (DR) vs uphill running (UR) at similar running speed or similar oxygen uptake (⩒O2). Eight well-trained, male, trail runners completed a maximal level incremental test and three 15-min treadmill running trials at ±15% slope: i) DR at ~6 km·h−1 and ~19% ⩒O2max (LDR); ii) UR at ~6 km·h−1 and ~70% ⩒O2max (HUR); iii) DR at ~19 km·h−1 and ~70% ⩒O2max (HDR). Cardiorespiratory responses and spatiotemporal gait parameters were measured continuously. Maximal isometric torque was assessed before and after each trial for hip and knee extensors and plantar flexor muscles. At similar speed (~6 km·h−1), cardiorespiratory responses were attenuated in LDR vs HUR with altered running kinematics (all p < 0.05). At similar ⩒O2 (~3 l·min−1), heart rate, pulmonary ventilation and breathing frequency were exacerbated in HDR vs HUR (p < 0.01), with reduced torque in knee (−15%) and hip (−11%) extensors and altered spatiotemporal gait parameters (all p < 0.01). Despite submaximal metabolic intensity (70% ⩒O2max), heart rate and respiratory frequency reached maximal values in HDR. These results further our understanding of the particular cardiorespiratory and muscular fatigue responses to DR and provide the bases for future DR training programs for trail runners. [ABSTRACT FROM AUTHOR]

Using detailed musculoskeletal models in computer simulations of human movement can provide insights into individual muscle and joint loading; however, these muscle models increase problem dimensionality and require difficult-to-fit parameters. Here, we provide a brief overview of a muscle model alternative, muscle torque generators (MTGs), and highlight how MTG functions have been used by researchers to generate accurate dynamic simulations of optimal sports performance. Multibody dynamic models of a golf drive, track cycling, and wheelchair propulsion were designed and actuated using MTGs. Each MTG was effectively a rotational, single muscle equivalent that contained joint angle/velocity scaling and passive elements to mimic Hill-type muscle model behaviour. Optimal control algorithms were used to predict how each model would execute their respective sports task; these results were compared against experimental data collected from elite athletes. Good agreement between simulated and experimental movement trajectories was observed, with relatively low computational times required for convergence of the MTG-driven multibody simulations. [ABSTRACT FROM AUTHOR]

BACKGROUND: Although several studies have shown an association of muscle weakness with gait speed (GS), no study has explored the relationship of muscle strength with swing phase duration and GS after stroke among the elderly in Saudi Arabia. OBJECTIVE: To examine the association of affected ankle dorsiflexor and hip flexor muscle strength with swing phase duration and GS in the elderly with different stroke chronicity. METHODS: In this cross-sectional study, we included a total of 60 post-stroke patients aged ≥55 years who were admitted in neurorehabilitation units between May 2017 and August 2018. Linear regression was employed to examine the association of muscle strength (measured using a handheld dynamometer) with swing phase duration and GS (both measured using the computerized Zebris–Mat). RESULTS: The chronicity of the stroke was negatively associated (p < 0.05) with swing phase duration. The ankle dorsiflexor muscle strength was significantly associated with GS (β= 0.656, p = 0.041). In contrast, hip flexor muscle strength was significantly associated with GS (β= 0.574, p < 0.0001) even after adjusting for stroke chronicity (β= 0.561, p < 0.0001). CONCLUSIONS: Stroke chronicity was the predictor that reduced swing phase duration. The ankle dorsiflexor muscle strength was associated with GS. However, the hip flexor muscle strength was associated with GS even after adjusting for stroke chronicity. [ABSTRACT FROM AUTHOR]

New Findings: What is the central question of this study?Does fukutin deficiency in skeletal muscle cause mitochondrial dysfunction, and if so, can AMP‐activated protein kinase (AMPK) stimulation via 5‐aminoimidazole‐4‐carboxamide ribonucleotide attenuate this through regulation of mitochondrial biogenesis and autophagy?What is the main finding and its importance?Mitochondrial dysfunction is associated with fukutin deficiency and AMPK stimulation may benefit muscle contractility to a greater extent than mitochondrial function. Disruptions in the dystrophin–glycoprotein complex (DGC) are clearly the primary basis underlying various forms of muscular dystrophies and dystroglycanopathies, but the cellular consequences of DGC disruption are still being investigated. Mitochondrial abnormalities are becoming an apparent consequence and contributor to dystrophy disease pathology. Herein, we demonstrate that muscle‐specific deletion of the fukutin gene (Myf5/fktn‐KO mice (Fktn KO)), a model of secondary dystroglycanopathy, results in ∼30% lower muscle strength (P < 0.001) and 16% lower mitochondrial respiratory function (P = 0.002) compared to healthy littermate controls (LM). We also observed ∼80% lower expression of the gene for peroxisome proliferator‐activated receptor‐γ coactivator 1α (PGC‐1α) (P = 0.004), a primary transcription factor for mitochondrial biogenesis, in Fktn KO mice that likely contributes to the mitochondrial defects. PGC‐1α is post‐translationally regulated via phosphorylation by AMP‐activated protein kinase (AMPK). Treatment with the AMPK agonist 5‐aminoimidazole‐4‐carboxamide ribonucleotide (AICAR) failed to rescue mitochondrial deficits in Fktn KO mice (P = 0.458) but did have beneficial (∼30% greater) effects on recovery of muscle contractility following injury in both LM and Fktn KO mice compared to saline treatment (P = 0.006). The beneficial effects of AMPK stimulation via AICAR on muscle contractile function may be partially explained by AMPK's other role of regulating skeletal muscle autophagy, a cellular process critical for clearance of damaged and/or dysfunctional organelles. Two primary conclusions can be drawn from this data: (1) fukutin deletion produces intrinsic muscular metabolic defects that likely contribute to dystroglycanopathy disease pathology, and (2) AICAR treatment accelerates recovery of muscle contractile function following injury suggesting AMPK signalling as a possible target for therapeutic strategies. [ABSTRACT FROM AUTHOR]

Prolonged low‐frequency force depression (PLFFD) induced by fatiguing exercise is characterized by a persistent depression in submaximal contractile force during the recovery period. Muscle glycogen depletion is known to limit physical performance during prolonged low‐ and moderate‐intensity exercise, and accelerating glycogen resynthesis with post‐exercise carbohydrate intake can facilitate recovery and improve repeated bout exercise performance. Short‐term, high‐intensity exercise, however, can cause PLFFD without any marked decrease in glycogen. Here, we studied whether recovery from PLFFD was accelerated by carbohydrate ingestion after 60 minutes of moderate‐intensity glycogen‐depleting cycling exercise followed by six 30‐seconds all‐out cycling sprints. We used a randomized crossover study design where nine recreationally active males drank a beverage containing either carbohydrate or placebo after exercise. Blood glucose and muscle glycogen concentrations were determined at baseline, immediately post‐exercise, and during the 3‐hours recovery period. Transcutaneous electrical stimulation of the quadriceps muscle was performed to determine the extent of PLFFD by eliciting low‐frequency (20 Hz) and high‐frequency (100 Hz) stimulations. Muscle glycogen was severely depleted after exercise, with a significantly higher rate of muscle glycogen resynthesis during the 3‐hours recovery period in the carbohydrate than in the placebo trials (13.7 and 5.4 mmol glucosyl units/kg wet weight/h, respectively). Torque at 20 Hz was significantly more depressed than 100 Hz torque during the recovery period in both conditions, and the extent of PLFFD (20/100 Hz ratio) was not different between the two trials. In conclusion, carbohydrate supplementation enhances glycogen resynthesis after glycogen‐depleting exercise but does not improve force recovery when the exercise also involves all‐out cycling sprints. [ABSTRACT FROM AUTHOR]

Speed skating is a cyclic sport which involves the hip abductor muscles, impelling the participant forwards, and adductor muscles, in the recovery phase and decelerating the abduction movement eccentrically. This paper has the objective of describing and comparing the abduction/adduction torque-angle curves of speed skating athletes (N=10) with a group of non-practitioners young participants (N=10). Both groups presented similar peak torques and electromyography patterns for tensor fascia lata, gluteus medius, long adductor and adductor magnus. However, athletes showed higher torque-angle curve integral and abduction and adduction peak torques at different hip angles than the control group. These findings suggest an adaptation of their musculotendon actuators and a better capacity to generate mechanical work and power during a propulsion-recovery cycle. [ABSTRACT FROM AUTHOR]

Backgroud: Femoral offset (FO) and limb length discrepancy (LLD) are important perioperative considerations when performing THA. Decreased FO prevents improvement of gait and muscle recovery and residual LLD has a prominent influence on patient satisfaction with THA, while few studies have investigated the relationship between FO and/or LLD and gait disturbances. We investigated the association between these two factors and hip muscle strength and the results of 3-D gait analysis after THA.Methods: We evaluated 92 patients (including 20 patients who underwent gait analysis) in whom total hip arthroplasty was performed for unilateral evere osteoarthritis of the hip joint. FO and LLD were measured on a standard anteroposterior radiograph of the pelvis. Hip muscle strength was evaluated by isometric hip flexion (in the manner of straight leg raising test: SLR) and hip abduction strength. To evaluate 3-D walking trajectory, we used a portable gait analyzer.Results: Reduction of global FO by > 5 mm after THA compared to the contralateral hip was associated with hip abductor muscle weakness. On the other hand, LLD ≤ 20 mm had no influence on hip abductor muscle strength and SLR strength. In gait analysis, SLR strength showed a significant difference between the sagittal plane symmetrical and asymmetrical groups.Conclusion: Postoperative global FO > 5 mm less than that of the contralateral hip was associated with hip abductor muscle weakness. And, from the results of 3-D gait analysis, SLR weakness may increase gait asymmetry in the sagittal plane. [ABSTRACT FROM AUTHOR]

Slips and falls are a serious public health concern in older populations. Reduced muscle strength is associated with increased age and fall incidence. Understanding the relationships between specific joint muscle strength characteristics and propensity to slip is important to identify biomechanical factors responsible for slip-initiated falls and to improve slip/fall prevention programs. Knee corrective moments generated during slipping assist in balance recovery. Therefore, the study goal was to investigate the relationship between knee flexion/ extension strength and slip severity. Isometric knee flexion/extension peak torque and rate of torque development (RTD) of the slipping leg were measured in 29 young and 28 older healthy subjects. Motion data were collected for an unexpected slip during self-paced walking. Peak slip velocity (PSV) of the slipping heel served as a slip severity measure. Within-sex and age group regressions relating gait speed-controlled PSV to strength of the slipping leg revealed significant inverse PSVknee extension peak torque and PSV-knee flexion/ extension RTD relationships in young males only. Differences in PSV-strength relationships between sex and age groups may be caused by greater ranges of strength capabilities in young males. In conclusion, the ability to generate higher, more rapid knee flexion/ extension muscle moments (greater peak torque/RTD) may assist in recovery from severe slips. [ABSTRACT FROM AUTHOR]

Abstract Background Therapeutic management of upper extremity (UE) function of boys and men with Duchenne Muscular Dystrophy (DMD) requires sensitive and objective assessment. Therefore, we aimed to measure physiologic UE function of healthy subjects and DMD patients in different disease stages, and to evaluate the relation between these physiologic measures and functional UE scales. Methods Twenty-three DMD patients and twenty healthy controls (7–23 years) participated in this explorative case–control study. Maximal muscle torque, maximal and normalized surface electromyography (sEMG) amplitudes, muscle thickness, echogenicity and maximal passive and active joint angles were measured. At activity level, Brooke upper extremity rating scale and the Performance of Upper Limb (PUL) scale were used. Results Outcome measures related to proximal UE function could discriminate between disease stages. Increased normalized sEMG amplitudes were found in patients, even in early disease stages. Maximal active joint angles showed the strongest relation to Brooke scale (R 2 = 0.88) and PUL scale (R 2 = 0.85). Conclusions The decline of muscle functions precedes the decline in performance of UE activities, and therefore may play a role in early detection of UE limitations. Increased sEMG levels demonstrate that DMD patients use more of their muscle capacity compared to healthy subjects, to perform daily activities. This might result in increased fatigability. Active maximal joint angles are highly related to functional scales, so preserving the ability to use the full range of motion is important for the performance of daily activities. Close monitoring of active joint angles could therefore help in starting interventions that minimize functional UE decline in DMD patients timely.

Purpose: The aim of this study was to evaluate the potential correlation between asymmetry of lower limb muscle torque, asymmetry of vertical ground reaction force during take-off in young soccer players and their jumping abilities. Methods: Twenty-three young soccer player (16.9±0.64 years old) participated in measurements. An isokinetic dynamometer, the Biodex System, was applied to test muscle torque (PT) of the knee flexors and extensors. The vertical ground reaction force (vGRF) was recorded from two Kistler plates. Jumping abilities were assessed with the horizontal (HJ) and vertical jump (VJ) tests. The asymmetry index (AI) was used to assess the asymmetry of the limbs. Results: The asymmetry index showed the highest asymmetry (over 10%) for the PT under static conditions for knee flexors and extensors. The correlation (-0.432, p=0.038) was found between the asymmetry of vGRF and the height of the VJ. There was no correlation between the muscle torque and the height of the vertical jump. However, a correlation between the HJ length and muscle torque for flexors and extensors of the right and left lower limb was found. Conclusions: The asymmetry of the muscle torque of the flexors and extensors of the knee joint does not correlate with the results of both jumping ability tests. There was a statistically significant correlation between the vGRF asymmetry index during take-off and the height of the VJ. In the HJ, such a relationship was not found. [ABSTRACT FROM AUTHOR]

The study aimed to analyse the effectiveness of two variants of 8-week strength training (hypertrophy, strength) with different modes of resistance. Healthy male subjects (n=75) were allocated to five groups of equal size: hypertrophy training with a variable cam (Hyp-Cam), hypertrophy training with disc plates (Hyp-Disc), maximal strength training with a variable cam (Str-Cam), maximal strength training with disc plates (Str-Disc), and a control group (CG). The Hyp-Cam and Str-Cam groups trained with a machine where the load was adjusted to the strength capabilities of the elbow flexors. The Hyp-Disc and Str-Disc groups trained on a separate machine in which a load was applied with disc plates. The CG did not train. All groups were assessed for changes and differences in one-repetition (1RM) lifts, isokinetic muscle torque, arm circumference and arm skinfold thickness, and plasma creatine kinase (CK) activity. Within the 8-week training period the 1RM increased (p<.001) in all groups by over 20%, without significant between-group differences. Muscle torque increased significantly (p<.001) only in the Hyp-Cam group (by 13.7%). Arm circumference at rest increased by 1.7 cm (p<.001) and 1.1 cm (p<.001) in the Hyp-Cam and Hyp-Disc groups, respectively, but not in the Str-Cam (0.3 cm; p>.05) or Str-Disc (0.2 cm; p>.05) group. Skinfold thickness of the biceps and triceps decreased more within the 8-week period in Str-Cam (by 1.1 and 2.1 cm; p<.001 and p<.001 respectively) and Str-Disc (0.7 and 1.5 cm; p<.001 and p<.01 respectively) than in Hyp-Cam (by 0.4 and 1.8 cm; p>.05 and p<.01 respectively) and Hyp-Disc groups (by 0.2 and 1.4 cm; p>.05 and p<.05 respectively). CK activity was significantly (p<.05) elevated in each training group except Hyp-Cam (p>.05). The 8-week hypertrophy training with a variable cam results in greater peak muscle torque improvement than in the other examined protocols, with an insignificant increase in training-induced muscle damage indices. [ABSTRACT FROM AUTHOR]

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Spinal cord injury (SCI) results in deleterious skeletal muscle adaptations, such as relevant atrophy and loss of force. In particular, the relevant loss of lower-limb force-generating capacity may limit functional mobility even if neuronal control was sufficient. Currently, methods of assessing maximal force-generating capacity using neuromuscular electrical stimulation (NMES) are limited in individuals who cannot tolerate higher stimulation amplitudes, such as those with residual sensation and those at risk of fracture. In this study, we examined the relationship between NMES amplitude and muscle torque exerted (recruitment curve) in order to determine whether maximal torque output can be characterized by a submaximal marker. Recruitment curves for knee extensors, knee flexors, and ankle plantarflexors were recorded from 30 individuals with motor complete SCI. NMES was delivered starting with an amplitude of 5 mA, and increasing by 5 mA for every subsequent stimulation until either the participant requested to stop the stimulation or the maximum stimulation amplitude (140 mA) was reached. Significant correlations between peak slope of the recruitment curve and peak torque for all muscle groups were found (knee extensors, r = 0.75; p < 0.0001; knee flexors, r = 0.68; p < 0.0001; ankle plantarflexors, r = 0.91; p < 0.0001), indicating that muscles that show greater peak slope of the recruitment curve tend to generate a greater peak torque. This suggests that peak slope, which was achieved at an average stimulation intensity (55.0 mA) that was 43% smaller than that corresponding to peak torque (97.4 mA), may be used as a submaximal marker for characterizing maximal torque output in individuals with SCI. [ABSTRACT FROM AUTHOR]

Objective:To investigate the relationship between electromyography (EMG) signal of the rectus femoris muscle and the muscle strength of the quadriceps at different torque levels of maximal isometric voluntary contraction (100%, 75%, 50%, 25%MVC) in normal young subjects.Methods:Five normal healthy young subjects were recruited in this study. The extension torque and surface electromyography (sEMG) of the right quadriceps muscles were measured during the four torque levels of isometric contraction, at 100%, 75%, 50%, 25%MVC.Results:There was a significant correlation between RMS values and the contraction force of quadriceps muscles (P

The torque-sharing strategies between synergistic muscles may have important functional consequences. This study involved two experiments. The first experiment (n 22) aimed 1) to determine the relationship between the distribution of activation and the distribution of torque-generating capacity among the heads of the hamstring, and 2) to describe individual torque-sharing strategies and to determine whether these strategies are similar between legs. The second experiment (n 35) aimed to determine whether the distribution of activation between the muscle heads affects endurance performance during a sustained submaximal knee flexion task. Surface electromyography (EMG) was recorded from biceps femoris (BF), semimembranosus (SM), and semitendinosus (ST) during submaximal isometric knee flexions. Torque-generating capacity was estimated by measuring muscle volume, fascicle length, pennation angle, and moment arm. The product of the normalized EMG amplitude and the torque-generating capacity was used as an index of muscle torque. The distributions of muscle activation and of torque-generating capacity were not correlated significantly (all P 0.18). Thus, there was a torque imbalance between the muscle heads (ST torque BF and SM torque; P 0.001), the magnitude of which varied greatly between participants. A significant negative correlation was observed between the imbalance of activation across the hamstring muscles and the time to exhaustion (P 0.001); i.e., the larger the imbalance of activation across muscles, the lower the muscle endurance performance. Torque-sharing strategies between the heads of the hamstrings are individual specific and related to muscle endurance performance. Whether these individual strategies play a role in hamstring injury remains to be determined. NEW & NOTEWORTHY The distribution of activation among the heads of the hamstring is not related to the distribution of torquegenerating capacity. The torque-sharing strategies within hamstring muscles vary greatly between individuals but are similar between legs. Hamstring coordination affects endurance performance; i.e., the larger the imbalance of activation across the muscle heads, the lower the muscle endurance. [ABSTRACT FROM AUTHOR]

Background: Many therapeutic uses for Pulsed magnetic fields have been extensively studied. Beneficial effects on the musculoskeletal system were established including bone healing, pain relief, and inflammatory conditions. The study aimed at investigating the possible effects of the pulsed magnetic field on quadriceps muscle torque, hamstring muscle torque and knee proprioception accuracy in athletic subjects. Methods: A randomized controlled trial which was carried out at the Faculty of Medical Rehabilitation Sciences, Taibah University, Saudi Arabia. Thirty healthy male athletic subjects, aging from 18 – 24 years were divided into two groups, study group (15 subjects) & control group (15 subjects). The pulsed magnetic field was applied for subjects of intervention group, while control group participants received sham pulsed magnetic field for 12 sessions over a period of 4 weeks. Quadriceps muscle torque, hamstring muscle torque & knee proprioception accuracy were measured before & after treatment for all participants using Biodex system 4 pro isokinetic dynamometer. Results: Multiway ANOVA was carried out to detect any significant differences within and between groups. Statistically significant differences between study and control groups were found after pulsed magnetic field application in quadriceps peak torque (at 60 and 120deg/sec) & hamstring peak torque (at 60 and 120deg/sec) and in knee proprioception accuracy (p-value was 0.004, 0.0001, 0.02, 0.03, 0.0001 respectively). Conclusion: Pulsed magnetic field has beneficial effects on quadriceps muscle peak torque, hamstring muscle peak torque and knee proprioception accuracy in healthy athletic subjects and can be advised as an adjunctive tool in rehabilitation programs.

Introduction: The aim of this study was to follow circulating brain-derived neurotrophic factor (BDNF) and interleukin-6 (IL-6) levels in response to severe muscle-damaging exercise.Methods: Young healthy men (N = 10) performed a bout of mechanically demanding stretch-shortening cycle exercise consisting of 200 drop jumps. Voluntary and electrically induced knee extension torque, serum BDNF levels, and IL-6 levels were measured before and for up to 7 days after exercise.Results: Muscle force decreased by up to 40% and did not recover by 24 hours after exercise. Serum BDNF was decreased 1 hour and 24 hours after exercise, whereas IL-6 increased immediately and 1 hour after but recovered to baseline by 24 hours after exercise. IL-6 and 100-Hz stimulation torque were correlated (r = -0.64, P < 0.05) 24 hours after exercise.Discussion: In response to acute, severe muscle-damaging exercise, serum BDNF levels decrease, whereas IL-6 levels increase and are associated with peripheral fatigue. Muscle Nerve 57: E46-E51, 2018. [ABSTRACT FROM AUTHOR]

Background. The aim of this study was to follow symptoms of exercise induced muscle damage in response to 50, 100 and 200 drop jumps with maximal intensity. Methods. Three groups of young healthy men (n = 10 in each group) performed a bout of mechanically demanding stretch shortening cycle exercise consisting of 50, 100 and 200 drop jumps. Voluntary and electrically induced knee extension torque, creatine kinase, muscle soreness were measured before and 24 h after exercise. Results. Indirect symptoms of exercise induced damage were dependent on DJs volume: the higher the numbers of jumps, the more symptoms were observed. Only after 200 DJs compared to 50-100 DJ there was decrease of CAR and H of DJ, and 24 h after 200 DJs CK was greater than after 50-100 DJs. Conclusion. In response to acute severe muscle-damaging exercise (after 200 DJs), indirect symptoms of exercise-induced muscle damage occurred. [ABSTRACT FROM AUTHOR]

Background: Gait speed declines with increasing age, but it is unclear if gait speed preferentially correlates with leg muscle strength or mass.Objective: We determined the relationship between gait speed and (1) leg muscle strength measured at 3 lower extremity joints and (2) leg lean tissue mass (LTM) in healthy young (age: 25 years, n = 20) and old (age: 70 years, n = 20) adults.Methods: Subjects were tested for maximal isokinetic hip, knee, and ankle extension torque, leg LTM by bioimpedance, and gait performance (i.e., gait speed, stride length) at preferred and maximal gait speeds.Results: We found no evidence for a preferential relationship between gait performance and leg muscle strength compared with gait performance and leg LTM in healthy young and old adults. In old adults, hip extensor strength only predicted habitual gait speed (R2 = 0.29, p = 0.015), whereas ankle plantarflexion strength only predicted maximal gait speed and stride length (both R2 = 0.40, p = 0.003).Conclusions: Gait speed did not preferentially correlate with leg muscle strength or leg LTM, favoring neither outcome for predicting mobility. Thus, we recommend that both leg muscle strength and leg LTM should be tested and trained complementarily. Further, hip and ankle extension torque predicted gait performance, and thus we recommend to test and train healthy old adults by functional integrated multiarticular rather than monoarticular lower extremity strength exercises. [ABSTRACT FROM AUTHOR]

Introduction. The aim of the study was to determine the changes in the peak torque of the knee extensors and flexors of the dominant lower limb, the shoulder internal and external rotators of the dominant upper limb, and the shoulder extensors and flexors of the dominant upper limb as well as the changes in jump height in volleyball players during the annual training macrocycle. Material and methods. The study involved 13 volleyball players from a Polish second-league team. The measurements were performed five times: before the preparation period (T1), at the beginning of the competitive season (T2), in the middle (T3) and at the end of the first competition period (T4) and after the competitive season (T5). The torque of the knee muscles and shoulder rotators was measured in isokinetic conditions and the torque of the shoulder extensors and flexors was assessed in isometric conditions. Jumping ability was tested using a piezoelectric platform. Results. We observed statistically significant differences (p < 0.05) in jump height and relative peak torque between the measurements, except for the torque of the shoulder external rotators and flexors. The results of multiple factor analysis based on 5 sets composed of 5 strength variables revealed differences between subjects and measurement sessions. Conclusions. The results obtained indicate that strength and jumping ability should be assessed regularly during the competitive season. The findings of the study suggest that it is necessary to modify the training methods used during the preparation period and individualize the training in the final phase of the competition period. [ABSTRACT FROM AUTHOR]

Background: Therapeutic management of upper extremity (UE) function of boys and men with Duchenne Muscular Dystrophy (DMD) requires sensitive and objective assessment. Therefore, we aimed to measure physiologic UE function of healthy subjects and DMD patients in different disease stages, and to evaluate the relation between these physiologic measures and functional UE scales.Methods: Twenty-three DMD patients and twenty healthy controls (7-23 years) participated in this explorative case-control study. Maximal muscle torque, maximal and normalized surface electromyography (sEMG) amplitudes, muscle thickness, echogenicity and maximal passive and active joint angles were measured. At activity level, Brooke upper extremity rating scale and the Performance of Upper Limb (PUL) scale were used.Results: Outcome measures related to proximal UE function could discriminate between disease stages. Increased normalized sEMG amplitudes were found in patients, even in early disease stages. Maximal active joint angles showed the strongest relation to Brooke scale (R 2  = 0.88) and PUL scale (R 2  = 0.85).Conclusions: The decline of muscle functions precedes the decline in performance of UE activities, and therefore may play a role in early detection of UE limitations. Increased sEMG levels demonstrate that DMD patients use more of their muscle capacity compared to healthy subjects, to perform daily activities. This might result in increased fatigability. Active maximal joint angles are highly related to functional scales, so preserving the ability to use the full range of motion is important for the performance of daily activities. Close monitoring of active joint angles could therefore help in starting interventions that minimize functional UE decline in DMD patients timely. [ABSTRACT FROM AUTHOR]

Purpose: Perception of action capabilities can be altered by changes in sensorimotor processes, as showed in previous works in populations dealing with regular and pathological sensorimotor deficits. Misestimating changes in performance ability could lead to risky behavior, injury, and/or reduced performance. However, the relationship between sensorimotor processes, the action-perception coupling, and the related anatomical structures is still a matter of debate. We investigated whether changes in the muscle-tendon system's mechanical properties experimentally induced by eccentric contractions could alter the action-perception coupling (APC) in a reaching-to-grasp task, in which the participants estimated the maximal distance they predicted that they would able to reach a glass.Methods: Based on their repartition, volunteers performed a conditioning session the first day: a series of isokinetic elbow extension in passive condition (control group, n = 11) or when performing elbow flexors eccentric contractions (eccentric group, n = 11). Performance estimates and actual performances in a reaching-to-grasp task were completed before, and immediately, 24 hours and 48 hours after the conditioning session. Alterations of musculo-articular mechanical properties were assessed through global joint stiffness (joint passive torque through load/unload cycles) and local stiffness (muscle elastography).Results: The results showed that the APC related to reaching-to-grasp performance was not impacted by post-exercise changes in mechanical properties of the musculo-articular system.Conclusion: These findings emphasize the central dimension of sensorimotor processing instead of peripheral structures to investigate the APC for an altered sensorimotor environment. [ABSTRACT FROM AUTHOR]

Background and purpose: Electrical stimulation (ES) has been used as one potential rehabilitative treatment option to restore motor function and improve recovery in patients with paresis, especially stroke patients. Multiple stimulation parameters affect the force response of muscle, creating a large number of possible ES parameter protocols. Within the available literature no optimal setting has been identified. So the purpose of the study was to compare the efficacy of different ES protocols on the isometric peak torque of quadriceps femoris muscle and level of discomfort in patients with stroke. Materials and Methods: Forty male patients having hemiparesis post stroke were recruited for the study from the neurological outpatient clinics of Cairo University hospitals. Patients were randomly assigned into two equal groups. Group (A) received the frequency modulation protocol of ES while group (B) received the Pulse duration modulation protocol of ES. Isometric peak torque (IPT) was evaluated before and after treatment, also the level of discomfort was measured immediately post treatment by Visual Analogue Scale. Results: There was a significant difference between both groups post-treatment in IPT and level of discomfort as (P=0.0001). The percentage of improvement in the IPT post treatment was 47.2% for group (A) while for group (B) was 26.27%. Conclusion: Electrical stimulation protocol that used variable frequency is an effective stimulation strategy in improving muscle performance than traditional approaches that used constant frequency with variable pulse duration. While the pulse duration modulation protocol produced greater muscle comfort. [ABSTRACT FROM AUTHOR]